A galaxy that’s all hat and no head

Hmph. I’m no genius, and I know there’s lots of astronomy-related things I don’t know that much about. But what surprises me is that there still are complete surprises for me… like a type of galaxy I’ve never heard of!

So here’s NGC 3621, as seen by the 2.2 meter MPG/ESO telescope in La Silla, Chile:

[Click to galactinate to the 3500 x 3100 pixel version.]

Pretty cool, right? This is a near true-color image, using three filters that come close to mimicking the eye’s blue, yellow-green, and red sensitivity, as well as a filter that selects the light from warm hydrogen gas (shown as pinkish-red). As usual, that last bit shows where stars are actively being born.

At 22 million light years away, NGC 3621 looks like your usual big spiral galaxy: flat disk, arms sweeping out majestically, central bulge… hey, hold on there a second. Where’s the central bulge?
Turns out, this galaxy doesn’t really have one. There’s a brightening to the center, sure, but no actual spheroidal region of old stars like in most spirals. That’s weird, and something I hadn’t heard of before! A galaxy that’s all disk.

Spirals can have all manners of central bulges. Andromeda, for example, has a nice puffy one. The Milky Way has a compact core but has a rectangular bar going across it. Some have huge bulges, some tiny. But I thought there always was one. But that’s not the case.

Turns out, current thinking is that central bulges in galaxies grow as galaxies collide. The galaxy itself grows, but gravitational instabilities in the collision lead to a build-up of stars in the center – and that forms the bulge. Apparently, though collisions are what made the Milky Way and Andromeda the giants they are today, NGC 3621 has never suffered through one, or at least not a big one. That’s a bit weird, but apparently more common than previously thought. Certainly more common than I thought, since I would’ve thought there’d be zero like that.

Another odd surprise too is that there’s evidence of a supermassive black hole in the center of this galaxy, and that it’s somewhat active– it’s weakly emitting energy as it gobbles down matter. All big galaxies have big black holes in their hearts (they form together, actually), but in general having an active one is something I would normally associate with galaxies that have cannibalized other galaxies. It doesn’t have to be that way, as NGC 3621 shows us, but still, it’s just another way this galaxy surprised me (even if it’s a mild one).

So look at that: I learned something, and not just anything: something surprising. That’s my favorite kind of thing to learn, too!